My mechanic is stymied! While driving - at any speed - my temperature gauge will rise to 3/4 or higher, stay there for approximately 2 seconds and then drop back to the expected temperature (slightly less than 1/2). This cycle will repeat over and over.

Any ideas how to fix this?

Details:
--The temperature seems to rise faster and stay hotter longer when I'm idling at a stop light or in traffic.
--I am losing coolant somewhere, but it has been pressure tested 3 times and there is not leak.
--I have had a timing belt/water pump/thermostat replaced and the issue persists.
--I have no coolant in the oil - the cap is clean.

After reading other posts, I'm still wondering about sensors for the coolant. My dad replaced the plug for the passenger side headlamp last October. He took the battery out to do that. My cooling/heating problems started in November. Could he have just knocked something?

I'm really tired of watching the temperature guage so closely when I drive! Any help or insights would be appreciated!!

I think you have a bad head gasket, that would explain both the loss of coolant and overheating issues.

^I'm in agreement Check your overflow, it's probably "overfilled". Also, check the link in my signature for lots of overheating information. Your answer is probably in there.

Thanks for the responses! I had read the "Overheating common causes - references" postings and several others. What I still don't understand is why when I keep driving, the temperature returns to normal? I can understand overheating and staying hot, but not the cycling between hot and normal.

Also, what is the overflow tank? I have been refilling coolant into the resevoir. I only have the owner's manual for the car and that is not very helpful for much besides radio functions.

I think I'm just desperate for it to not be the head gasket because I don't want to drop that much $$!

"Overflow" is the plastic tank attached to the radiator by a hose:

It's possible that the head gasket isn't completely destroyed, only partially; you lose a bit of coolant, the temperature spikes and once more coolant circulates the temperature comes back down. I wish I could tell you it's something minor, but your losing coolant slowly and there's no leak in the system, which means the engine must be consuming it..

Because, you esentially have air in the system. The displaced coolant has to be replaced by something, thats where the air comes in. The coolant flow all but stops when idling if theres air in the system. Rev it up when leaving the stop and more coolant instantly circulates. I know its not heater weather, but I bet you loose heat at an idle too. Sounds like head gasket problems. Have a shop sniff it with a sniffer to tell if the coolant has any carbons in it. Hands down this is the very best way to tell if you have head gasket problems. IF you do it will show up, if you don't it won't. If not look elsewhere, if so fix it. leaning toward it sense the other big items are already done. The only thing that may be the cause too is that they didn't get all the air out when they did all the other stuff.

I had this same problem. Replaced a ton of crap and dumped a ton of money into it only to find out it was a bad radiator cap.

Would just like to say your guys are awesome!! Thanks for the responses.

My overflow was overfilled. Pulled some coolant out and I thought my problems were solved. Nope. It started getting hot again on Wednesday. I bought a new rad cap last night and I'm hoping that's it. The temp was good and steady all the way to work this morning.

stick with honda parts if you start replacing coolant related items like radiator cap and thermostat. a wrong or cheap radiator cap can cause serious damage to the cooling system and water pump. always do the necessary small items first before jumping to headgasket being bad. make sure your radiator fan is turning on when you are stopped at idle and its hot outside. if it never comes on, it could explain overheating when not moving. when moving, air is blowing thru the radiator and that is enough to keep it cool. when stopped, there is no air movement.

Sounds like a head gasket. I recently had the same issue and it will get worse over time. I hope it was your rad cap as suggested earlier. I had to replace the head gasket. I did it myself and it took a few weeks and probably took a few years off my life. Good luck.

^ LOL head gasket job isn't THAT bad

Definitely sounds like the head gasket.. I just dealt with the same problem. It will persistently get worse and the longer you drive it in that condition the worse off your car will be.

Check the dates my friend.

Finally problem solved after 3 months of this same issue of 3/4 temp gauge and getting hot problem…
first I’d like to say I changed everything and I mean everything on the engine trying to solve this problem radiator new Honda cap 3 different thermostats every temp sensor and head gasket timing belt water pump still ran at 3/4 and then would cool down then 3/4 then cool down and if you drove for more than 40 minutes it would go to hot still after everything being new mind blowing for 3 months so did another pressure test to see if it was head gasket again to make sure it wasn’t bad and maybe warped head and everything checked out good now mind blowing again so decided to start over with thought process pulled hoses off and attached garden hose to every hose to see if flow was good in and out of all hoses through engine water flowed great from what I could tell so hooked everything back up now my thought just by seeing water was wonder if the new water pump was pumping so I pulled heater hose off because it’s before thermostat and in my mind figured no matter what it should pump water out if you start car so I did and low and behold had hardly anything come out so now I’m in thought of how the heck a new water pump would be bad so ordered a new kit under warranty of course and commenced to round 2 of timing belt and water pump so took apart went to Autozone to pick up parts with the old parts in hand and by chance compared the water pumps and there it was plain as day the Impellers were noticeably different the one I had brought back in the cone on the impeller that goes into the block tapered sleeve was deeper which would almost choke the water off from block now after seeing it I was getting a little better feeling that was the problem but still not sure till it was together so low and behold fired it up it pumped out the hose so put hose back on filled all the way bleed system and back to normal temperature everything great…. So answer is even new parts can be wrong or junk so inspect the impeller 🤷‍♂️🤷‍♂️
and forget to say it was doing all the same things as the above comments and problems

This is why we recommend only using honda parts